This invention relates to a process for the production of adsorbents of nitrohumic acid series with an improved alkali-resistance. More particularly, this invention relates to a process for the production of adsorbents of nitrohumic acid series with an improved alkali-resistance wherein calcium nitrohumate insolubilized by a heat treatment is used as active ingredient.
Nitrohumic acid produced by oxidation of lignite, brown coal and vituminous coal with nitric acid is well known and is used in a wide variety of fields such as fertilizers and deodorants. This compound is a high molecular weight substance ranging in molecular weight from a few to several ten thousands and as it possesses acidic groups such as carboxyl or phenol groups in the molecule, it has a chemical nature of capturing metal ions.
In recent years, environmental pollutions caused by mining and industrial effluents, created big problems. Among them, pollutions by metal-containing effluents are serious and present actual problems awaiting solution. We intended to utilize the aforementioned chemical nature of nitrohumic acid to remove or to recover metal ions, especially heavy metal ions, from metal-containing solutions including such effluents and have carried out researches for the practical application of nitrohumic acid.
As a result of such researches, it was found previously that an adsorbent with a strong activity to remove metal ions contained in the polluted effluents and ample practical strength to be easily regenerated and used repeatedly, can be produced by shaping into a desired form a composition comprised of sodium nitrohumate and a small amount of water-soluble polyacrylamide having a molecular weight of at least 5,000,000 and then subjecting the shaped article to a heat treatment conducted between 120.degree. C and 160.degree. C followed by an acid treatment [Japanese Pat. Appln. No. 32476 /Showa 48(1973 )]. On the other hand, our researches revealed that the adsorbent produced in this manner has some defects; when the adsorbent is applied to alkaline effluents having a pH above 10.5, a part of nitrohumic acid is dissolved in the effluents, resulting in coloration of the treated water and gradual deterioration of its own nature. Thus, this adsorbent having a poor resistance to alkali was found still unsatisfactory for practical use. Accordingly, there is a great demand for adsorbents which can be applied without difficulty to various kinds of effluents including acidic and alkaline ones for removing therefrom heavy metals.